An air-fuel-ratio sensor and an oxygen sensor, which detect the concentration of oxygen within exhaust gas, are known as gas sensors, and are used for improving the fuel consumption of internal combustion engines such as automotive engines and/or for performing combustion control for the engines. There has been a desire for a reduction in the amount of nitrogen oxides (NOx) within exhaust gas in order to cope with the strengthened exhaust gas regulation for automobiles, and NOx sensors which can directly measure NOx concentration have been developed. Such a gas sensor includes a sensor element, and the concentration of a specific gas is detected on the basis of the output from the sensor element. A known structure of such a sensor element is such that the sensor element assumes a platelike shape and includes at least one cell composed of an oxygen-ion conductive solid electrolyte layer formed of zirconia or the like, and a pair of electrodes formed on the surface of the solid electrolyte layer.
FIG. 9 shows the structure of a gas sensor (hereinafter, may be simply referred to as a “sensor”) 1000 which includes a plate-shaped sensor element 100. The sensor 1000 is an assembly into which the sensor element 100 is assembled. Sensor 1000 includes the sensor element 100, and a metallic shell 200 for mounting the sensor 1000 to an object (e.g., an exhaust pipe of an automotive engine). The metallic shell 200 assumes an approximately cylindrical shape and has an inner hole 280 formed therein. The metallic shell 200 holds the sensor element 100 within the inner hole 280 such that a front end portion (a detection portion 110) of the sensor element 100 projects from the metallic shell 200. The sensor element 100 is held within the metallic shell 200 via a collar unit 450. The collar unit 450 is fixed to the sensor element 100 at a predetermined position along the longitudinal direction of the sensor element 100. The collar unit 450 is brought into engagement with a step portion 290 provided in the inner hole 280 of the metallic shell 200. Notably, the collar unit 450 is composed of a tubular metallic cup 205 which has, at its front end, an abutment portion 215 for abutment against the step portion 290; a ceramic holder 210 accommodated within the metallic cup 205; and a first fixing member (specifically, powder of talc) 220 which is compressively charged into the metallic cup 205 to form a layer on the ceramic holder 210, to thereby provide a seal between the outer surface of the sensor element 100 and the inner surface of the metallic cup 205.
In a state where the collar unit 450 is engaged with the step portion 290, a second fixing member (specifically, powder of talc) 230 and a ceramic sleeve 300 are placed within the inner hole 280 so as to surround the sensor element 100. Subsequently, a rear end portion of the metallic shell 200 is crimpled radially inward, with a metallic packing 310 disposed between the rear end portion and the ceramic sleeve, so as to compress the charged second fixing member 230, to thereby hold (fix) the sensor element 100 within the metallic shell 200.
Further, in the gas sensor 1000, an outer sleeve 800 is joined to the outer circumference of a rear end portion of the metallic shell 200, and lead wires 680 connected to electrodes of the sensor element 100 via terminal electrodes extend outward from the rear end of the outer sleeve 800 via a grommet 770. Meanwhile, an outer protector 400 and an inner protector 410, which are formed of metal and which cover the detection portion 110 of the sensor element 100, are attached to the outer circumference of a front end portion of the metallic shell 200.
Since ceramic layers, such as solid electrolyte layers, which constitute the sensor element 100 are brittle, the sensor element 100 may crack or break at the time of assembly of the sensor 1000 or during shipment of the sensor 1000, whereby the sensor element becomes defective. In view of this, there has been proposed a method of detecting a defect of the sensor 1000 by means of introducing a pressurized gas into the sensor 1000. Further, a technique has been proposed in order to detect a growing crack of the sensor element 100. (See Japanese Patent Application Laid-Open (kokai) No. 2006-30140). The proposed technique makes use of a phenomenon that, if the sensor 1000 has a crack, a portion of the sensor 1000 displaces when the sensor 1000 is pressed, and detects the crack from a change rate of the pressing load of the sensor 1000.
In the case of the gas sensor 1000 configured such that the first and second fixing members 220 and 230 provide a seal against the outer surface of the sensor element 100 and a seal against the inner surfaces of the tubular metallic members (the metallic cup 215 and the metallic shell 200), even when a portion of the sensor element 100 surrounded by the first and second fixing members 220 and 230 breaks, detection of such a defect is difficult.
In general, when the sensor element 100 breaks, each of paired conductors, which are provided on a solid electrolyte layer and extend through the portion surrounded by the first and second fixing members 220 and 230, also breaks. Therefore, by means of detecting electrical discontinuity between the paired conductors, occurrence of breakage, such as two-piece breakage, of the sensor element 100 can be determined. However, in the portion of the sensor element 100 surrounded by the first and second fixing members 220 and 230, even when sensor element 100 breaks, the first and second fixing members 220 and 230 support the sensor element 100 through the close contact of fixing members 220 and 230 with the outer surface of the sensor element 100. Therefore, even when the sensor element 100 breaks, in some cases, no gap is formed between portions of the conductors physically separated from each other through the breakage, and contact therebetween is maintained. In such a case, the above-mentioned electrical discontinuity cannot be detected as a continuity defect. In some cases, such a defective sensor 1000 may be shipped and attached to an object to which the sensor is to be attached, such as an exhaust pipe, for use. In such a case, during use, the breakage of the conductors may become noticeable due to vibration, thermal expansions of members, or other causes, and a continuity defect may be discovered.
In view of the forgoing, an object of the present invention is to provide a defect detection method for a sensor in which a fixing member providing a seal between a sensor element and a tubular metallic member is disposed to surround the sensor element, the method being capable of detecting breakage of a conductor caused by, for example, two-piece breakage of the sensor element.